Moon and Gemini

StarDate: April 2, 2009

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The Moon scoots through Gemini tonight. The constellation is best known for its bright "twin" stars, Pollux and Castor. They're above the Moon as darkness falls.

Just below the Moon is one of the other stars that form the pattern of the twins, Delta Geminorum. It's a bit hotter and brighter than the Sun, but a good bit farther along in life. In fact, it's beginning to move into old age. As it does so, it'll puff up like a balloon, getting cooler but brighter. It'll resemble Pollux, which has already entered that late stage of life.

Another "puffy" star is close by. But R Geminorum doesn't stay puffed up all the time. Instead, it expands and contracts like a balloon that's repeatedly inflated and deflated. It takes a bit longer than a year to complete one cycle.

When R Geminorum puffs up, it gets hundreds of thousands of miles wider. That makes it several hundred times brighter.

R Geminorum is a little farther along in its evolution than Delta Geminorum. In fact, the nuclear "engine" in its core is just about to shut down. At some point, the star's outer layers will puff out and just keep on puffin' -- they'll head out into space. For awhile, that will create a beautiful nebula around the star. But when the nebula fades away, only the star's core will remain. This small, hot cosmic ember will glow feebly for billions of years as it slowly cools and fades from sight....

McDonald Observatory will host an open house on Saturday, with free tours and other activities. Details are on the web at

Script by Damond Benningfield, Copyright 2009

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